These studies are concerned with the neuroendocrine, neurobiochemical, and immunological consequences of neonatal hydrocortisone treatment during maturation. This treatment mimics one component of the consequences of neonatal stress in the rat and intrauterine stress of higher animals which are born in a more mature state than this rodent. Hormone administration at a time when mechanisms regulating many physiological functions are very primitive and are just beginning to develop is expected to redirect the normal course of events. Experiments involving development of the diurnal rhythmicity of the pituitary-adrenal system and associated changes in central biogenic amines, development of the stress response and the adrenocortical feedback system, development of growth hormone uptake by the brain, and development of thymic function by assaying responses to antigens participating in either humoral or delayed hypersensitivity will be included.